Vehicle capacitive detection systems comprising capacitive sensors operated in loading mode are nowadays widely used, for instance for the purpose of detection of vehicle seat occupancy. The capacitive sensors may be designed as sense-only capacitive sensors having a single sense electrode. Alternatively, they may be designed as guard-sense capacitive sensors, having a sense electrode and a guard electrode proximally arranged and mutually insulated from each other.
For example, document JP-H11-78655 describes a vehicle seat occupancy detecting apparatus including electric field sensors. A high frequency oscillator whose frequency is about 100 kHz is connected via a resistor to an antenna electrode arranged on an automobile seat. By this, a differential AC electric field is generated between the antenna electrode and the automobile ground, so that a load current corresponding to the AC electric field flows through the resistor. The AC load current is converted by the resistor into an AC voltage which is then transmitted by a voltage buffer to a detector including a bandpass filtering function which generates a DC output voltage.
If an occupant is seated on the seat, the current flowing between the antenna electrode and the automobile ground is increased, indicating the presence of the occupant on the seat.
Further, in the field of measuring technology employing sigma-delta modulators is commonly known, and numerous relevant literature is available on this topic, for instance: “Sigma-Delta ADCs and DACs”, AN-283 Application Note, Analog Devices, 1993.
U.S. Pat. No. 8,570,053 B1 describes an application of a sigma-delta modulator for measurements of unknown capacitance values by a capacitive sensor. The capacitive sensor includes a switching capacitor circuit, a comparator, and a charge dissipation circuit. The switching capacitor circuit reciprocally couples a sensing capacitor in series with a modulation capacitor during a first switching phase and discharges the sensing capacitor during a second switching phase. The comparator is coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response. The charge dissipation circuit is coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.